This application claims the priority of 001 23 326.1, filed in Germany, and corresponding application filed in the European Patent Office under European Application No. EP00123326.1, filed in Europe on Oct. 27, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method of operating a hot-chamber diecasting machine by which molten metal is pressed from the casting vessel by way of an ascending bore, a mouthpiece and a feed orifice into a mold. The invention also relates to a hot-chamber diecasting machine by means of which this method can be implemented.
In the case of the hot-chamber method, the liquid metal is delivered by way of a casting vessel and a casting plunger into a mold. The casting vessel and the casting plunger are, in this case, constantly situated in the metal bath. During the movement of the plunger and also at the end of the plunger movement, depending on the temperature of the molten metal, losses occur between the plunger rings and the casting vessel bore. Therefore, in the case of the hot-chamber method, when casting zinc, which has a metal bath temperature of approximately 420xc2x0 C., approximately 300 bar of metal pressure can be generated at the end of the filling operation. When pressure casting magnesium, which has a metal bath temperature of approximately 650xc2x0 C., only approximately 250 bar of metal pressure can be reached also at the end of the filling operation.
Cold-chamber diecasting methods (German Patent Document 29 22 914 C2) also exist by which the mold filling phases take place in a manner similar to that of the hot-chamber diecasting method. In the cold-chamber method, in which the casting vessel and the casting plunger are not situated in the liquid molten metal, it is possible to generate higher end pressures of a magnitude of from 400 bar to 700 bar. This means that, because of the high metal pressure of the cold-chamber method, it is possible to produce parts of a higher density. This means, in turn, that there is less porosity in the diecast part, as well as a high stability, higher elongation values and a higher surface density.
In the case of the hot-chamber diecasting method, the filling operation of the mold takes place approximately in 7 ms to 20 ms (milliseconds). As mentioned above, the maximal casting pressure is built up at the end of the filling operation. By way of the feed orifice, this casting pressure acts upon the metal already situated in the mold cavity. Since the thickness of the feed orifice is a function of the wall thickness and of the surface quality of the parts as well as of the finishing, and the thinnest wall thickness of the feed orifice is the thickness of the gate, the molten metal will first solidify at this point. As a result, the feed orifice is closed off from the mold cavity, and the afterpressure applied from the direction of the casting plunger can no longer be effective or can no longer be fully effective. For the purpose of an explanation, it is pointed out that the thinnest wall thickness of a gate, for example, in the case of a zinc part, is in the range of 0.3 to 0.6 mm and, in the case of a magnesium part, is in the range of 0.4 to 0.8 mm. As a result of the cooling occurring in this area, the material solidifies relatively fast at this point.
It is an object of the present invention to provide, in the case of a method of the initially mentioned type that, despite the lower end pressures of the hot-chamber casting method, diecast parts can be achieved which have similar characteristics as those produced by the cold-chamber method.
For achieving this object, it is suggested in the case of a method of the initially mentioned type that, at the end of the mold filling operation, a compressional vibration, which prevents the molten metal from rapidly solidifying, is generated at least in the narrowest cross-section of the feed orifice. By varying the pressure, a movement is achieved in the molten metal which has the result that the previously mentioned gate cross-section with its thin wall thickness will not solidify so fast and thus does not xe2x80x9cfreezexe2x80x9d. In this manner, the pressure can act into the mold for a longer time and can therefore also counteract the volume-caused shrinking of the molten metal.
As a further development of preferred embodiments of the invention, the pressure can be increased after a certain time period by way of a time function element, in which case the pulsation is maintained so that, when the molten metal has reached the so-called semisolid phase, the highest densification will occur. In this phase, no more burr will occur on the outer contours of the diecast part. As a result of the vibrations, which can be introduced at a relatively high frequency, the pressure is fully transmitted to the metal situated in the mold. This will result in a sort of hammering upon the filled mold which leads to a final densification of the material.
As a further development of certain preferred embodiments of the invention, in the case of a method in which a casting plunger is present which is moved by way of an electric-motor-operated drive, the pulsating pressure can be generated by superimposing a vibration upon the drive. As a further development of certain preferred embodiments of the invention, this vibration may amount to approximately 300 Hz and can be introduced at a defined deceleration of the casting plunger velocity. The casting plunger velocity can be determined in the known manner as a function of the path so that it will not be problematic to determine the point in time at which the pulsating pressure becomes necessary.
As a further development of certain preferred embodiments of the invention, the pressure can be decreased or increased in a pulsating manner compared with the maximal casting pressure, in which case, as previously indicated, the pressure in the end phase is decreased during a first short time period and is increased during a second time period before the complete solidification of the molten metal occurs.
The invention also relates to a hot-chamber diecasting machine by means of which the new method can be implemented. This hot-chamber diecasting machine has a casting plunger drive and a control device therefor. A pulsation device, which can be connected in the end phase of the filling operation and whose vibrations act upon the drive shaft of the casting plunger, is assigned to the casting plunger drive. If the casting plunger drive is equipped with a casting plunger driven by an electric motor, the pulsation device may consist of an electric servo drive and of a control device acting upon the latter. This control device may be an electronic computer which is operated as correspondingly designed software. The servo drive itself may be a brushless electric motor with a low flywheel effect. Such a drive largely avoids the effect of moments of inertia upon the casting plunger which, however, in a known manner, can also be reduced by means of an elastic element between the driving motor and the casting plunger or by a controlled limiting of the servo drive.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.